This invention relates to new hot melt adhesives having a high setting rate and a low melt viscosity.
Reactive, moisture hardening hot melt adhesives from the class of isocyanate functional prepolymers are known, for example from DE-OS 2 4OI 320, EP A 0 107 097 or EP-A-O 125 009. Adhesives of the type mentioned in these prior publications are applied to substrates as hot melts and solidify on cooling and thus rapidly build up strength so that further processing of the substrates can be carried out within a short time. The final strength is obtained by the complete reaction of the free isocyanate groups still present, for example with atmospheric moisture, to form high molecular weight polyurethane polyureas. It is particularly advantageous to apply reactive hot melt adhesives without solvents. The disadvantage of the adhesives described in the aforesaid publications lies particularly in their high melt viscosity. For optimum wetting of a surface with the adhesives it is desirable to obtain as low a melt viscosity as possible.
The adhesives according to U.S. Pat. No. 5,019,638 also require a relatively high temperature to reduce the viscosity of the adhesive to a sufficiently low value for wetting surfaces. The adhesives are still found to have viscosities of from 7000 to 20,000 mPa.s at 130.degree. C. and therefore cannot be used for bonding temperature sensitive substrates.
In view of the very short operating cycles nowadays customary in industry, it is necessary for the hot melt adhesives to set very rapidly by crystallization after their application so that the bonded substrates are immediately ready for further processing.
German Patent Application P 39 13 406.7 in the name of the present Applicants deals with prepolymers of the type which are preferred according to the invention, but it only deals with their use for the preparation of moisture hardening coating compounds or sealing compounds. The use of the prepolymers as hot melt adhesives or for the preparation of hot melt adhesives is not described in the Application which is, therefore, not a prior publication.
DE-OS 1 668 069 describes ester group-containing polyisocyanates which are said to be suitable as adhesives. However, since the polyisocyanates of this prior publication, as can be seen from the examples of embodiments, are based on a completely different type of low molecular weight polyhydroxyl compounds, they differ fundamentally in their melt characteristics from the polyisocyanates used according to the invention. This conclusion is justified in that the authors of the said prior publication have not taken into account hot melt adhesives of the type to be used according to the present invention.
It was, therefore, an object of the present invention to provide new reactive hot melt adhesives which would combine the advantages of a high setting speed with a low melt viscosity.
This problem has been solved by the use, according to the invention, of certain polyisocyanates containing ester groups described below.